Implantable medical devices have been used to obtain various types of dynamic and non-dynamic impedance measurements. Examples of dynamic impedance measurements include low frequency impedance Zo (sometimes also referred to as raw impedance, or low frequency raw impedance), respiratory impedance Zr, and cardiogenic impedance Zc (sometime also referred to as cardiac impedance). A lead impedance measurement is an example of a non-dynamic impedance measurement. These various types of impedance measurements have been used for many different types of applications. For example, cardiogenic impedance signals have been used for monitoring hemodynamic stability, performing arrhythmia discrimination, prediction and monitoring of heart failure progression, and functioning as a hemodynamic (such as stroke volume) surrogate. For another example, respiratory impedance signals have been used to monitor respiratory rate and respiratory volume. However, impedance signals typically have various limitations. For example, the amplitude of an impedance signal is typically relatively small, the signal is typically susceptible to noise, and the signal typically is susceptible to changes in activity and body posture. Particularly, while a single impedance vector may have a good signal-to-noise ratio, it may have a small signal amplitude variation, or poor morphology consistency across different subjects (i.e., patients). In addition, the good and bad aspects of a vector may not be consistent across different subjects. For example, a vector with good signal variation in one patient may not yield good signal variation in another patient. Therefore, it is difficult to select a single impedance vector that will have consistent characteristics for various patients and applications.
FIG. 1 includes an upper plot of an exemplary cardiogenic impedance signal, and a lower plot of a corresponding intracardiac electrogram (IEGM) signal. Referring to the upper plot, the exemplary cardiogenic impedance signal has an overall consistency across patients, but is noisy and has very small variations for some patients. In addition, in order to obtain a representative morphology and reduce overall noise, an adequate number of beats may need to be collected, for example, 50-100 beats, which may require up to 2 minutes. Continuing with this example, if measurements from four different vectors are being made, up to 8 minutes may be required (i.e., up to 2 minutes per vector, for each of four vectors). As compared to other diagnostics features or data collection techniques, this is extremely time consuming. Therefore, a method to collect several impedance vector signals substantially simultaneously would be very beneficial.
FIGS. 2A and 2B will now be used to illustrate how prior art impedance measurement and processing circuitry 202 can be used to measure one impedance vector at a time using an exemplary maximum sampling rate of 128 Hz. More specifically, FIG. 2A is a high level block diagram illustrating the impedance measurement and processing circuitry 202, and FIG. 2B is a corresponding timing diagram that is used to explain the operation and limitations associated with the circuitry 202 shown in FIG. 2A.
Referring to FIG. 2A, the prior art impedance measurement and processing circuitry 202 includes of a current pulse generator 204 and a current pulse multiplexer 206, which can also be referred to as an output multiplexer. The multiplexer 206 connects to all of the electrodes (patient nodes) in the system such that impedance can be measured for any electrode combination. More specifically, the output of the multiplexer 206 is connected to electrode terminals 208, which are electrically connected to implantable electrodes by lead conductors. The impedance measurement and processing circuitry 202 also includes a voltage measurement multiplexer 216, which can also be referred to as an input multiplexer. Additionally, the impedance measurement hardware includes a sensing circuit 220, a signal processing channel 240, and an analog-to-digital converter (ADC) 250. The sensing circuit 220 is shown as including an amplifier 222 and an integrator 224. The signal processing channel 240 is shown as including a sample-and-hold (S/H) circuit 242 and three parallel switched-capacitor filters 244a, 244b and 244c (base impedance, cardiogenic impedance and respiratory impedance filters). Using the impedance measurement and processing circuitry 202, an impedance measurement can be obtained, e.g., by sending out a current pulse between any two (or more) electrodes in the system while measuring the resulting voltage area between any two (or more) electrodes. Since the current area is known, an impedance measurement can be obtained by dividing the voltage area by the current area.
Since the impedance measurement and processing circuitry 202 only supports measurement of one vector at a time, multi-vector (or multi-channel) measurements need to be sequential. When switching to a new vector (which is done by controlling the output and input multiplexers 206 and 216) the sudden change from one impedance vector signal to another will cause an impulse response for up to several seconds in the filters 244. So, in addition to the time consuming sequential vector measurements, there is also a delay when switching between different vector measurements. This can be appreciated from the discussion of FIG. 2B below.
Referring to FIG. 2B, assuming a pulse rate and sampling rate of 128 Hz (which corresponds to one pulse generated, and one sample obtained, each 7.81 ms), and assuming that there is a desire to obtain fifty (50) impedance measurements for each of four (4) different vectors, then it takes a total of 383 ms to obtain 50 impedance measurements for the first vector (Vector 1), a total of 383 ms to obtain 50 impedance measurements for the second vector (Vector 2), a total of 383 ms to obtain 50 impedance measurements for the third vector (Vector 3), and a total of 383 ms to obtain 50 impedance measurements for the fourth vector (Vector 4). If the filters 244a, 244b, 244c of the signal processing channel 240 had an instantaneous impulse response, then the circuitry 202 can switch between Vector 1 and Vector 2 in 7.8 ms, between Vector 2 and Vector 3 in 7.8 ms, and between Vector 3 and Vector 4 in 7.8 ms, enabling 50 impedance measurements for each of the four Vectors to be obtained in 1.56 seconds, i.e., (383 ms×4)+(7.8 ms×3)=1.56 seconds. However, in actuality, since switching from one vector to another causes an impulse response for up to several seconds in the filters 244, after switching from one vector to the another vector there is a need to wait a relatively long time (e.g., at least 2 seconds, i.e., at least 2000 ms) after switching from one vector to another vector before beginning to obtain impedance measurements for the temporally later vector. Accordingly, it would actually take at least 6.56 seconds to obtain 50 impedance measurement for each of the four Vectors, i.e., (383 ms×4)+(7.8 ms×3)+(3×2 seconds)=6.56 seconds. For certain types of applications, this would be acceptable, e.g., if the measurements were being used to monitor the integrity of leads and/or electrodes. However, for other types of applications, some of which are discussed below, that total amount of time necessary to obtain the desired impedance measurements would be much too long.